In response to the demands of consumers who are driven both by ever-escalating fuel prices and the dire consequences of global warming, the automobile industry is starting to embrace the need for ultra-low emission, high efficiency cars. While some within the industry are attempting to achieve these goals by engineering more efficient internal combustion engines, others are incorporating hybrid or all-electric drive trains into their vehicle line-ups.
In recent years, electric vehicles (EVs) have proven to be not only environmentally friendly, but also capable of meeting, if not exceeding, consumer desires and expectations regarding performance, range, reliability, and cost. In order to insure both vehicle reliability and user safety, electric vehicles use a variety of techniques to prevent battery pack abuse as well as mitigate the effects of an unavoidable abusive event (e.g., battery pack damaged during a collision, etc.). Fuses, which may be employed at the battery level, the battery pack level, or both, are one of the primary means of protecting an EV's battery pack. Unfortunately while fuses may be used to provide very effective protection in a low current circuit, due to the high current levels common in an EV the response time of a fuse may be too slow to provide the desired level of protection. This phenomenon is illustrated in FIG. 1 which provides the cutoff current characteristics for a variety of conventional high current fuses ranging from a 300 amp fuse to an 800 amp fuse. As expected, as the current rating of the fuse increases, so does the time it takes to blow the fuse for a given current level. For example for this set of exemplary fuses, a 300 amp fuse subjected to 1000 amps of current will take approximately 8 seconds to blow while a 600 amp fuse may take as much as 200 seconds to blow at the same current level. Subjecting an EV's electrical system to an overcurrent of such magnitude and for such an extended period of time may damage the battery pack as well as any of a variety of other system components (e.g., motor, motor controller, accessory systems, etc.). To avoid this problem, the fuse within an EV's power train may be undersized, thus insuring that the fuse will blow quickly. Unfortunately undersizing the fuse may also lead to it blowing prematurely during routine vehicle use, albeit typically only under extreme conditions (e.g., high temperature conditions).
Accordingly, what is needed is a system that allows the use of an undersized fuse in order to provide rapid response to excessive currents while still insuring that the fuse will not blow during normal vehicle operation. The present invention provides such a system.